Heat oil and butter in a large saucepan over medium-high heat, add onion and garlic and sauté until tender (3-4 minutes). Add chicken stock, season to taste, bring to the boil and cook for 5 minutes for flavours to develop, then add peas and simmer until just tender (3-4 minutes). Remove from heat, take out about ½ cup of peas with a slotted spoon (reserve for garnish), then add spinach, mint, crème fraîche, rind and juice to remaining pea mixture. Blend with a hand-held blender until smooth, check seasoning and serve hot with a spoonful of crème fraîche, reserved peas and extra mint.